Zarys treści w języku angielskim: A reliable simulation of a rainfall-runoff process depends on the credibility of the parameters in the model used, which could be affected by natural changes in the water regime in the basin. The water regime in most basins is affected by changes in vegetation cover, and not just in the annual cycle, but also over many decades. Then, the resultant evapotranspiration, a phenomenon important for the proper identification of model parameters, depends on the actual climate situation as well as changes in soil moisture, which are affected by vegetation. The differences between observed and simulated flows over a reasonably long period of time can be considered to be an indicator of change in a water regime, where changes in evapotranspiration demand are significant.